1. Field of the Invention
The present invention relates to mobile communications, and more particularly to a method of informing uplink synchronization acquisition in a mobile communication system by using downlink transmit power control (TPC) bits so that a terminal can be informed that uplink synchronization has been obtained.
2. Background of the Related Art
In general, RRC (Radio Resource Control) states are defined to distinguish the operational modes and communication states of a terminal (e.g., mobile station, subscriber unit, user equipment (UE), etc.). The RRC states can be broadly defined into an idle mode and a connected mode. When an RRC connection is established between the terminal and the UTRAN (Universal Terrestial Radio Access Network), for example, the terminal transitions from the idle mode to the connected mode. As shown in FIG. 1, the connected mode can be classified into four states based upon the types of physical channels that the terminal can use: CELL_DCH (cell dedicated channel), CELL_FACH (cell forward access channel), CELL_PCH (cell paging channel), and URA_PCH (URA paging channel) states. Here, each RRC state can transition to another RRC state or to a different mode. The four types of states under the connected mode will now be considered in more detail.
First, the CELL_DCH is a state in which a dedicated channel (DCH), necessary for continuous data transmissions, is allocated. Here, a downlink shared channel (DSCH) can also be used. The CELL_FACH is a state for monitoring the forward access channel (FACH), in which a terminal monitors all FACHs being transmitted thereto. Also, the CELL_PCH and URA_PCH are states for monitoring a paging channel (PCH). The CELL_PCH and the URA_PCH are distinguished from each other based on whether a cell is to be updated or whether a URA (UTRAN Registration Area) is to be updated.
Accordingly, rapid data transmission requires a rapid transition from the CELL_FACH to CELL_DCH, and during this transition, transport channels and physical channels are reconfigured. However, for transition from the CELL_FACH to CELL_DCH, namely, for establishing one dedicated channel (DCH), synchronization of the uplink and downlink channels between the terminal and the base station needs to occur.
In general, in a WCDMA radio communication system, the terminal (e.g., mobile station, user equipment (UE), etc.) searches the downlink signal transmitted from the base station (e.g., Node B) to obtain downlink synchronization and to establish an initial call with the network. Once the synchronization of the downlink is acquired, the terminal transmits a power control preamble for a certain amount of time to achieve network timing, namely, to allow the base station to acquire uplink synchronization, while the base station searches for uplink synchronization.
As previously explained, the terminal searches the downlink signal transmitted from the base station to obtain a downlink synchronization, and then transmits a preamble for power control for a certain amount of time such that the base station may obtain uplink synchronization. Thus, because the terminal can only transmit data frames after a preamble transmission for performing power control has been completed, the terminal must wait a certain amount of time to transmit/receive data, even if the base station has already obtained uplink synchronization before the preamble transmission has been completed.